1. Field of the Invention
The present invention relates to a top-blown converter and, more particularly, to both a lance structure, which is to be used with the top-blown converter for blowing oxygen from the top into molten iron contained in the converter thereby to refine the molten iron into steel, and a process for blowing oxygen together with slag-forming flux in a powdered form into the molten iron.
2. Description of the Prior Art
In the oxygen top-blown steel making technique, as is well known in the art, slag-forming agent or flux is added to react with the impurities contained in pig iron so that slag may be formed to effectively promote dephosphorization. In accordance with that steel making technique, moreover, a refining coverter is charged with the pig iron, scrap and a sub-material, and an oxygen gas is blown to penetrate into the converter from an oxygen lance so that the pig iron may be refined into steel. Here, if the sub-material, i.e., the slag-forming flux such as quicklime, fluorite, dolomite or iron ore is in a powdered form, it will be scattered by the carbon monoxide gas generated as a result of the refining reaction. In order to prevent this, the converter is charged with the slag-forming flux in an agglomerated form. Nevertheless, it is still difficult to completely melt the quicklime or lime stone within a blowing time period thereby to promote formation of the slag because the quicklime or lime stone is composed mainly of CaO having a high melting point of about 2570 C. In other words, it is difficult to form the slag thereby to effectively promote dephosphorization and desulphurization.
In order to eliminate that difficulty, there has been developed the so-called "LD-AC process (or OLP process)". According to this process, the quicklime powder acting as the slag-formed flux is premixed with the oxygen gas so that it may be carried by the oxygen gas to penetrate into the top surface of the molten iron contained in the converter. This process is advantageous in that the flux can be scattered in the oxygen gas flow and can be carried to penetrate directly into a fire point which is formed by the oxygen jet. As a result, the flux is promptly heated by the molten iron to react with the impurities in the iron so that the slag is formed to promote the dephosphorization and desulphurization. However, since the quicklime powder premixed with the oxygen will wear and damage a Laval nozzle which is used to generate a supersonic jet of the oxygen gas for increasing the depth of penetration, the velocity of the oxygen jet is dropped to produce a so-called "soft blow". As a result, much FeO is formed, and slopping phenomena frequently occur to make the running operations difficult or drop the production yield. Moreover, the lance lifetime is considerably shortened. Hence, the process of the prior art has not been put into actual practice partly because there is required a system for premixing the flux powder with the oxygen gas flow under a high pressure so that the cost for the facilities inclusive is raised and partly because the steel making efficiency of the process is not satisfactory.
In order to overcome those disadvantages of the foregoing process, there has also been proposed a process in which the oxygen lance is equipped with a flux feeding nozzle in addition to the oxygen nozzle so that the oxygen jet injected from the oxygen nozzle may cross, downstream of the lance, the slag-forming flux spurting from the flux feeding nozzle together with the carrier gas and may be blown into the molten iron. Nevertheless, the process thus proposed in the art is effective to prevent the Laval nozzle of the oxygen lance from being worn. In case the process is applied to a large-sized converter, the carrier gas has to be fed at a flow rate sufficient for effectively dispersing the powder in the oxygen gas jet so that the cost for the piping system of the converter is raised to a remarkably high level. In case the existing converter is to have its construction changed, on the other hand, the process under consideration is liable to be restricted in its facilities. If the oxygen gas jet is directed to cross the carrier gas jet carrying the slag-forming flux, the blowing operation has a tendency to become "hard", as is well known in the art, so that spitting phenomena become so intense as to cause loss of the iron material itself.